Composite Passivation Process for Nitride FET

ABSTRACT

A nitride-based FET device that provides reduced electron trapping and gate current leakage. The device includes a relatively thick passivation layer to reduce traps caused by device processing and a thin passivation layer below the gate terminal to reduce gate current leakage. The device includes semiconductor device layers deposited on a substrate. A plurality of passivation layers are deposited on the semiconductor device layers, where at least two of the layers are made of a different dielectric material to provide an etch stop. One or more of the passivation layers can be removed using the interfaces between the layers as an etch stop so that the distance between the gate terminal and the semiconductor device layers can be tightly controlled, where the distance can be made very thin to increase device performance and reduce gate current leakage.

GOVERNMENT CONTRACT

This invention was made with Government support under Contract No.N00014-05-C-0121 awarded by the Office of Naval Research. The Governmentmay have certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a nitride-based field effecttransistor (FET) device employing a plurality of dielectric passivationlayers and, more particularly, to a nitride-based FET device includingat least two dielectric passivation layers deposited on semiconductordevice layers, where two of the passivation layers are made of differentmaterials so that an interface between the layers acts as an etch stopto accurately control the distance between a gate terminal and thesemiconductor device layers.

2. Discussion of the Related Art

Due to the wide bandgap and high carrier saturation velocity,nitride-based FET devices are ideal for high frequency and high powerapplications. However, these devices have had limited performancebecause they can suffer from electron trapping near the device surfaceand high gate terminal current leakage.

Dispersion or current collapse caused by electrons trapped at thesurface of the semiconductor device near the gate terminal edge reducesthe achievable power performance of nitride-based FET devices. Inaddition, under high bias conditions, the high electric fields near thegate terminal edge during device operation can cause trap formation andincreases dispersion during device operation leading to prematuredegradation of the power performance. One widely accepted model fordispersion is that electrons injected into trap states near the gateterminal form an extended virtual gate near the gate terminal edgeextending the depletion region of the device channel. Since the responseof the depletion region relies on electrons being removed from trapstates, the device will not respond as fast as the depletion regionunder the gate terminal. This effectively results in reduced power ofthe device under high frequency operation.

It has been reported in the literature that device dispersion can bereduced by depositing a dielectric passivation layer, such as siliconnitride (SiN), over the device after ohmic and gate contacts have beenformed. Typically, without the passivation layer deposited in the accessregions between the source and gate contacts and the gate and draincontacts, devices can experience nearly total current collapse or 100%dispersion. Studies have shown that dispersion can be reduced byoptimization of the surface preparation before passivation layerdeposition and by the quality of the passivation deposition itself.Other studies have shown that by using a SiN first process where SiN isdeposited before fabrication process, the dispersion can be reducedcompared to depositing the SiN after the gate and ohmic contacts havebeen formed. The improvement in performance has been attributed to theprotection the passivation layer in the SiN first process affords thesurface during device fabrication. Furthermore, other studies have shownthat utilizing a SiN first process where the SiN is deposited in-situwithout exposing the semiconductor layers to the air environment nearlyeliminates dispersion completely. This body of evidence indicates thattraps effecting dispersion occur at or near the surface of the accessregion of the device. This evidence also indicates that processingsteps, such as ohmic anneals, plasma cleans, etc., can induce traps inthe unprotected device surface.

High gate leakage current reduces power performance and can lead topremature failure an FET device. Nitride-based FET devices typicallysuffer from high gate leakage due to extended defects in a barrier layerof the device or traps along the surface of the FET device.

Some prior art nitride-based FET devices have utilized a MISFET typestructure where a thin dielectric layer is left under the gate terminalto reduce gate current leakage problems. The dielectric layer increasesthe barrier to tunneling and reduces the gate current leakage. Inaddition it has been shown that utilizing a thin SiN dielectric layerunder the gate terminal can improve reliability of the device anddrastically improve the gate current stability of the device.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, anitride-based FET device is disclosed that provides reduced electrontrapping and gate current leakage. The device includes a relativelythick passivation layer in the access regions of the device to reducetraps caused by device processing and a thin passivation layer below thegate terminal to reduce gate current leakage. The device includessemiconductor device layers deposited on a substrate. A plurality ofpassivation layers are deposited on the semiconductor device layers,where at least two of the layers are made of a different dielectricmaterial to provide an etch stop. The passivation layers may be fully orpartially removed in the areas of the device designated for source anddrain terminals so that the source and drain terminals can be formeddirectly on the semiconductor device layers. The passivation layers maybe left intact or may be partially removed in the area of the devicedesignated for a gate terminal. One or more of the passivation layerscan be removed using the interfaces between the layers as an etch stopso that the distance between the gate terminal and the semiconductordevice layers can be tightly controlled, where the distance can be madevery thin so that device performance is not severely impacted, butsufficiently thick to reduce gate current leakage.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor device profileemploying passivation layers, according to an embodiment of the presentinvention;

FIG. 2 is a cross-sectional view of an FET semiconductor deviceemploying passivation layers between source and drain terminals,according to an embodiment of the present invention;

FIG. 3 is a cross-sectional view of an FET semiconductor device showinga passivation layer having been etched down to accommodate a gateterminal, according to an embodiment of the present invention;

FIG. 4 is a cross-sectional view of an FET semiconductor device showingtwo passivation layers having been etched down to accommodate a gateterminal, according to another embodiment of the present invention;

FIG. 5 is a cross-sectional view of an FET semiconductor device showingthe source terminal and the drain terminal positioned against aplurality of passivation layers, according to another embodiment of thepresent invention;

FIG. 6 is a cross-sectional view of an FET semiconductor device showinga metal layer between the source terminal and the passivation layers andthe drain terminal and the passivation layers, according to anotherembodiment of the present invention; and

FIG. 7 is a cross-sectional view of an FET semiconductor device showingdielectric material deposited between the source terminal and thepassivation layers and the drain terminal and the passivation layers,according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toan FET device employing passivation layers where one of the layers actsas an etch stop for a gate terminal of the device is merely exemplary innature, and is in no way intended to limit the invention or itsapplications or uses. For example, the present invention is applicablefor many different types of FET and other semiconductor devices as willbe appreciated by those skilled in the art.

FIG. 1 is a cross-sectional view of the material profile of asemiconductor device 10, according to an embodiment of the presentinvention. FIG. 1 is intended to show a partial fabrication step of thedevice 10, where the device 10 can be any suitable nitride-based fieldeffect transistor (FET) device, such as high electron mobilitytransistor (HEMT) devices, metal semiconductor field effect transistor(MESFET) devices, metal oxide field effect transistor (MOSFET) devices,metal insulator field effect transistor (MISFET) devices, modulationdoped field effect transistor (MODFET) devices, etc. The device 10includes a substrate 12 made of any suitable material for anitride-based FET device, which is typically sapphire, SiC, Si, AIN orGaN. Semiconductor device layers 14 are deposited on the substrate 12.The device layers 14 are intended to represent device layers for any ofthe FET devices mentioned above, and depending on the particular device,can be one or more of buffer layers, nucleation layers, channel layers,barrier layers, cap layers, etc., all of which are well understood tothose skilled in the art.

According to the invention, at least two dielectric passivation layersmade of different materials are deposited on the device layers 14. Inthe non-limiting embodiment shown in FIG. 1, the passivation layersinclude a first passivation layer 16 deposited on the device layers 14,a second passivation layer 18 deposited on the first passivation layer16, and a third passivation layer 20 deposited on the second passivationlayer 18. The passivation layers 16, 18 and 20 are intended to beprotective layers that protect the semiconductor device layers 14 fromexposure to surface damage during the device fabrication process, whichmay include plasma etches, chemical cleans, high-temperature anneals,etc., so that damage to the device layers 14 does not occur that couldaffect device performance. According to the invention, any suitableprocess can be used to deposit the layers 14, 16, 18 and 20 on thesubstrate 12, such as molecular beam epitaxy (MBE) processes, chemicalvapor deposition (CVD) processes, physical vapor deposition (PVD)processes, atomic layer deposition (ALD) processes, or any suitabledeposition process for nitride-based FET devices.

Also, according to the invention, all of the layers 14, 16, 18 and 20can be deposited on the substrate 12 as a continuous process so that thesemiconductor device layers 14 are not exposed to air, or any otherdamaging fabrication steps. However, in an alternate embodiment, thelayers 16, 18 and 20 are deposited on the substrate 12 after the devicelayers 14 have been exposed to air. In either of these two processes,the dielectric layers 14, 16, 18 and 20 are deposited prior to source,drain or gate terminals being formed.

In one non-limiting embodiment, the first passivation layer 16 is a thindielectric layer, such as silicon nitride (SiN). Typically, thethickness of the first passivation layer 16 will be in the range of5-150 Å, but the layer 16 can have a greater thickness in certainembodiments, such as 5-300 Å. The second passivation layer 18 is also athin dielectric layer, but is made of a different dielectric material,such as aluminum nitride (AIN), than the first passivation layer 16. Thethickness of the layer 18 is typically in the range of 10-250 Å. As willbe discussed in detail below, the interface between the firstpassivation layer 16 and the second passivation layer 18 operates as anetch stop, where a suitable wet or dry etchant that will dissolve thematerial of the second passivation layer 18, but will not dissolve thematerial of the first passivation layer 16 so that the distance betweenthe semiconductor device layers 14 and a gate terminal can be tightlycontrolled.

The third passivation layer 20 is a relatively thick layer, typically inthe range of 10-1000 Å, and can be made of a dielectric material that isthe same as the second passivation layer 18 or different than the secondpassivation layer 18 depending on whether the interface between thesecond passivation layer 18 and the third passivation layer 20 needs tooperate as an etch stop for gate terminal distance control. The thirdpassivation layer 20 is made relatively thick so that the distancebetween the top of the third passivation layer 20 and the top of thedevice structure layers 14 is large enough so that processing stepsafter the passivation layers 16, 18 and 20 have been deposited will notdamage the device layers 14 creating traps.

FIG. 2 is a cross-sectional view of the semiconductor device 10including a source terminal 24 and a drain terminal 26. The passivationlayers 16, 18 and 20 have been etched by suitable etchants that willremove portions of the passivation layers 16, 18 and 20 to create viaswhere the source terminal 24 and the drain terminal 26 can be depositeddirectly on the semiconductor device layers 14. Depositing contacts inthis manner for the source terminal 24 and the drain terminal 26requires high temperature annealing and other fabrication steps thatcould damage the device layers 14, possibly causing trapping. However,the active portion of the device layers 14 between the source and drainterminals 24 and 26 are protected by the passivation layers 16, 18 and20. In an alternate embodiment, a portion of the passivation layers 16,18 and 20 is left between the source and drain terminals 24 and 26 andthe device layers 14.

In this embodiment, the source terminal 24 and the drain terminal 26 areshown spaced from the passivation layers 16, 18 and 20. This may be doneto prevent interactions between the contact metal and the passivationlayers 16, 18 and 20 during the high temperature processing required tocreate the contacts.

In another embodiment, the passivation material for the layers 16, 18,and 20 that do not interact with the source and drain contact metalsduring the contact process may be used to eliminate the space betweenthe terminals 24 and 26 and the passivation layers. This embodiment isshown in FIG. 5 for an FET device 40 where the source terminal 24 andthe drain terminal 26 are positioned against the passivation layers 16,18 and 20.

In another embodiment, metal may be deposited in the space between thesource terminal 24 and the passivation layers 16, 18 and 20, and thedrain terminal 26 and the passivation layers 16, 18 and 20 after thehigh temperature contacts of the terminals 24 and 26 have been created.This embodiment is shown in FIG. 6 for an FET device 42 where a metallayer 28 is deposited between the source terminal 24 and the passivationlayers 16, 18 and 20, and a metal layer 30 is deposited between thedrain terminal 26 and the passivation layer 16, 18 and 20.

In another embodiment, a dielectric material can be deposited in thespace between the source terminal 24 and the passivation layers 16, 18and 20 and the drain terminal 26 and the passivation layers 16, 18 and20 after the high temperature contacts of the terminals 24 and 26 havebeen created. This embodiment is shown in FIG. 7 for an FET device 44where a dielectric layer 46 is deposited between the source terminal 24and the passivation layer 16, 18 and 20, and a dielectric layer 48 isdeposited between the drain terminal 26 and the passivation layers 16,18 and 20.

It is desirable that the distance between the gate terminal and thesemiconductor device layers 14 be as small as possible to increasedevice performance. However, it is not desirable to provide the gateterminal directly on the device layers 14 because this can lead to highgate current leakage. Therefore, it is desirable to provide a thindielectric layer between the gate terminal and the device layers 14.

FIG. 3 shows an embodiment of the present invention where a suitablemask has been used to etch a via through the third passivation layer 20to create an opening for a gate terminal 38. In this embodiment, thematerial of the passivation layer 20 and the passivation layer 18 wouldbe different so that the interface between the layers 18 and 20 createsan etch stop so that the distance between the bottom surface of the gateterminal 38 and the top surface of the device layers 14 can be tightlycontrolled. In this embodiment, the passivation layers 16 and 18 aremade of different materials, but do not need to be, and can be a singlethicker layer made of the same material, but different than thepassivation layer 20. The etch time required to etch the via in thepassivation layer 20 does not need to be tightly controlled because oncethe etchant used to etch the material of the passivation layer 20reaches the material of the passivation layer 18, the etching will stop.In other words, instead of relying on time to stop an etch for thesetypes of devices, which was done in the prior art, the present inventionstops the etch by using an etchant that only etches the material that isto be removed, and not the material of the underlying layer.

As discussed above, the gate recess for the gate terminal 38 is formedby a low damage etch process that has sufficient selectivity to stop ata specific material interface within the plurality of dielectric layers.Alternately, the gate recess can be formed by more than one etch processthat has sufficient selectivity to remove multiple layers within theplurality of dielectric layers 16, 18 and 20.

Thus, the present invention provides three benefits, namely a thickprotective layer is provided above the access regions between the sourceterminal 24 and the gate terminal, and the drain terminal 26 and thegate terminal 38, which reduces dispersion and improves reliability.Also, a thin dielectric layer is provided under the gate terminal 38that reduces gate current leakage and improves reliability.

FIG. 4 is a cross-sectional view of a device 36, according to anotherembodiment of the present invention, where a via is formed through thepassivation layers 18 and 20 for the gate terminal 38, and where thedistance between the gate terminal 38 and the top of the device layers14 is set by the thickness of the passivation layer 16. The passivationlayer 16 can be quite thin, such as 10 Å, so that the gate terminal 38is very close to the device layers 14. In this embodiment, the materialof the second passivation layer 18 and the first passivation layer 16are different so that the interface therebetween acts as an etch stop inthe manner as discussed above. The passivation layers 18 and 20 can bemade of the same material or a different material, where if thepassivation layers 18 and 20 were made of the same material, a singlelayer could be used to form the two passivation layers 18 and 20. Thepassivation layers 16 and 20 can be made of the same material and thepassivation layer 18 would be a different material. In this embodimenttwo selective etches might be used to define the gate recess. The firstwould stop at the interface between the layers 20 and 18. The secondselective etch would be designed to stop at the interface between thelayers 16 and 18.

The device fabrication process could include an overall passivationlayer deposition that protects the device after the terminals 24, 26 and38 have been formed. The FET device 44 shown in FIG. 7 also shows suchan overall passivation layer 32 in combination with the passivationlayers 46 and 48 that protect the entire device 44.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A field effect transistor device comprising: a substrate; a pluralityof semiconductor device layers deposited on the substrate; a pluralityof dielectric passivation layers deposited on the semiconductor devicelayers; a source terminal deposited directly on and in contact with thesemiconductor device layers; a drain terminal deposited directly on andin contact with the semiconductor device layers; and a gate terminaldeposited on at least one of the passivation layers, wherein at leasttwo of the passivation layers are made of a different dielectricmaterial and wherein the thickness of the passivation layers between thesource terminal and the gate terminal and the drain terminal and thegate terminal is greater than the thickness of the one or morepassivation layers between the gate terminal and the semiconductordevice layers so that passivation layers are provided at sides of thegate terminal.
 2. The device according to claim 1 wherein the pluralityof passivation layers is three passivation layers where the thickness ofthe combination of two of the passivation layers closest to the devicelayers is thinner than a top passivation layer on the two passivationlayers.
 3. The device according to claim 2 wherein the gate terminal isdeposited only on a passivation layer closest to the device layers. 4.The device according to claim 2 wherein the gate terminal is depositedon both of the two passivation layers closest to the device layers. 5.The device according to claim 1 wherein the thickness of the one or morepassivation layers between the gate terminal and the device layers is inthe range of 5-150 Å.
 6. The device according to claim 1 wherein atleast one of the passivation layers is silicon nitride and at least oneof the passivation layers is aluminum nitride.
 7. The device accordingto claim 1 wherein the device is a nitride-based device.
 8. The deviceaccording to claim 7 wherein the substrate is selected from the groupconsisting of sapphire, SiC, Si, AIN and GaN substrates.
 9. The deviceaccording to claim 1 wherein the device is selected from the groupconsisting of HEMT devices, MESFET devices, MOSFET devices, MISFETdevices and MODFET devices.
 10. The device according to claim 1 whereinthe semiconductor device layers and the passivation layers are depositedon the substrate by a process selected from the group consisting ofmolecular beam epitaxy processes, chemical vapor deposition processes,physical vapor deposition processes and atomic layer depositionprocesses.
 11. The device according to claim 1 wherein the semiconductordevice layers and the passivation layers are deposited by the sameprocess.
 12. The device according to claim 1 wherein the passivationlayers are deposited before the semiconductor device layers are exposedto air.
 13. The device according to claim 1 wherein the semiconductordevice layers and the passivation layers are deposited by differentprocesses.
 14. The device according to claim 1 wherein the sourceterminal and the drain terminal are deposited directly on thesemiconductor device layers.
 15. A nitride-based field effect transistordevice comprising: a substrate; a plurality of semiconductor devicelayers deposited on the substrate; three dielectric passivation layersdeposited on the semiconductor device layers, and where a middle one ofthe passivation layers is made of a different material than the othertwo passivation layers; a source terminal deposited directly on and incontact with the semiconductor device layers; a drain terminal depositeddirectly on and in contact with the semiconductor device layers; and agate terminal deposited on only one of the passivation layers closest tothe semiconductor device layers so that the middle and top passivationlayers are provided at sides of the gate terminal.
 16. The deviceaccording to claim 15 wherein the thickness of the passivation layerbetween the gate terminal and the device layers is in the range of 5-150Å.
 17. The device according to claim 15 wherein at least one of thepassivation layers is silicon nitride and at least one of thepassivation layers is aluminum nitride.
 18. The device according toclaim 15 wherein the source terminal and the drain terminal aredeposited directly on the semiconductor device layers.
 19. Anitride-based field effect transistor device comprising: a substrate; aplurality of semiconductor device layers deposited on the substrate;three dielectric passivation layers deposited on the semiconductordevice layers, where a middle one of the passivation layers is made of adifferent material than the other two passivation layers; a sourceterminal deposited directly on and in contact with the semiconductordevice layers; a drain terminal deposited directly on and in contactwith the semiconductor device layers; and a gate terminal deposited ontwo of the passivation layers closest to the semiconductor device layersso that a top passivation layer is provided at sides of the gateterminal.
 20. The device according to claim 19 wherein the thickness ofthe two passivation layers between the gate terminal and the devicelayers is in the range of 5-150 Å.
 21. The device according to claim 19wherein at least one of the passivation layers is silicon nitride and atleast one of the passivation layers is aluminum nitride.
 22. The deviceaccording to claim 19 wherein the source terminal and the drain terminalare deposited directly on the semiconductor device layers.